The present invention relates to a planetary gear train for an automatic transmission.
FIG. 1 shows a known planetary gear train manufactured by General Motors in United States. The planetary gear train comprises two planetary gear sets G1, G2, four clutches, C1, C2, C3, C4, two brakes B1, B2, and two one-way clutches OWC1, OWC2. A first sun gear S1 is selectively connected with an input shaft I via the clutch C3 and the one-way clutch OWC1, a first ring gear R1 is connected with a second pinion carrier PC2, a first pinion carrier PC1 is connected with a second ring gear R2 and also with an output shaft O, the first ring gear R1 is selectively connected with the input shaft I via the clutch C4, a second sun gear S2 is seletively connected with the input shaft I via the clutch C2, the first sun gear S1 is selectively connected with the input shaft I via the clutch C1, the second sun gear S2 is adapted to be achored by the brake B1, the second carrier PC2 is adapted to be anchored by the brake B2 and the one-way clutch OWC2 prevents reverse rotation of the second pinion carrier PC2. The friction devices C1 to C4, B1, B2, OWC1, and OWC2 are engaged or released according to a pattern shown in FIG. 2 to establish four speeds during D-range, one reverse speed and two speeds during L-range.
The above planetary gear train, however, is encountered with problems as follows. The first problem is that the stress of each tooth of the sun gears S1 and S2 is large so the allowable upper limit of the input torque is low. This is attibuted to the fact that the sun gears have the smallest diameters among all planetary rotary elements, and the input shaft I drives the first sun gear S1 via the clutch C3 and the one-way clutch OWC1 in first, second and third speeds, and it drives the second sun gear S2 via the clutch C2 in the reverse drive. The second problem is that the sun gear S1 rotates at about 2.5 times the rotation of the input shaft I in fourth speed, so that the strength of a member that is connected with this sun gear S1 has to be increased accordingly and this member requires a bearing construction that has a long endurability. The third and last problem is that a portion of the rotation fed to the planetary gear set G1 from the input shaft circulates between the two planetary gear sets G1, G2, causing a drop in transmission effeciency of the power and besides, this arrangement is not preferrable from the standpoint of endurability of the planetary gear sets.